In countries where river improvements are not sufficient, smooth evacuation from flooding is important for decreasing loss of life and properties. However, the development of flood forecasting and warning system has not advanced properly because of the financial difficulty, the lack of rainfall data. The authors are developing the fundamental tool for flood forecasting system using GIS data and rainfall data observed by satellites. Although it is necessary to improve about the accuracy of the runoff calculation using these data, it is found that these data are applicable to flood forecasting from verification results of runoff calculation at some basins. This system implements the interface to input satellite-based rainfall data, the GIS modeling function, the default runoff analysis model, and the output interface to display results, which enables runoff calculation necessary for flood forecasts and alerts rapidly and effectively.